Optical fibers are in widespread use today as transmission media because of their large bandwidth capabilities and small size. However, they are mechanically fragile, exhibiting low strain fracture under tensile loading and degraded light transmission when bent. Accordingly, cable structures have been developed to protect mechanically the optical fibers thereby rendering them a realizeable transmission medium.
Developments in the optical fiber communications field have been rapid. However, the technology still is undergoing major shifts in direction. For example, earlier generation fiber systems were designed to operate at wavelengths of about 0.8 .mu.m, and current systems operate at 1.3 .mu.m. Now there is growing interest in systems having an operating wavelength of about 1.55 .mu.m to take advantage of the loss window that exists in silica-based optical fiber in that wavelength region. Another example of a major shift which is driven by demand for higher bandwidths is that from multimode to single mode fibers.
Although desired for their large bandwidth capabilities and small size, light-transmitting optical fibers are mechanically fragile, exhibiting low-strain fracture under tensile loading and degraded light transmission when bent. The degradation in transmission which results from bending is known as microbending loss. As a result, cable structures have been developed to protect mechanically the optical fibers in various environments.
A cable for use in a duct must be capable of withstanding tensile loads applied when the cable is pulled into the duct and stresses caused by bends. Cable structures which have been developed for optical fibers include loose tube, stranded and ribbon cables. For a description of loose tube cables, see, for example, D. Lawrence and P. Bark "Recent Developments in Mini-Unit Cable" published at pp. 301-307 of the Proceedings of the 32nd International Wire and Cable Symposium, 1983. See also U.S. Pat. No. 4,153,332.
Ribbon cable comprises a core including one or more ribbons with each including a plurality of optical fibers disposed generally in a planar array. The core is surrounded by a loose-fitting plastic inner tubular jacket. In one ribbon cable, a plastic outer jacket is reinforced with strength members which are encapsulated in the outer jacket to achieve coupling therewith.
In some situations, especially duct systems which include many bands such as those in loop plant in urban areas, relatively high tensile loads are expected. An improved optical communications cable which is suitable for such use is disclosed in U.S. Pat. No. 4,241,979 which issued on Dec. 30, 1980 in the names of P. F. Gagen and M. R. Santana. A bedding layer, about which strength members are wrapped helically, is added between plastic extruded inner and outer jackets to control the extent to which the strength members are encapsulated by the outer jacket. The cable includes two separate layers of strength members, which are wrapped helically in opposite directions. Under a sustained tensile load, these two layers of strength members produce equal but oppositely directed torques about the cable to insure the absence of twisting.
In another type of optical communications cable, a plurality of optical fibers are enclosed in an extruded plastic tube to form a unit and a plurality of these tubed units are enclosed in a common extruded plastic tube which is enclosed in a sheath system. Generally, the optical fibers which are enclosed in each unit tube are stranded together about a central strength member. A central strength member is used because it is relatively easy to assemble into the cable. Also, the cable is more easily bent if it has a central strength system rather than strength members which are incorporated into the sheath system. However, when such a cable is bent, the central strength member may in some instances compress one or more of the fibers against the tube and causes damage thereto.
Generally, optical fiber cables of the prior art, such as stranded and loose tube, suffer from the disadvantage of having the stranded units or the tubes manufactured on a separate line. In stranded cable, for example, a plurality of units which priorly have been enclosed individually in tubes and stranded are fed into a line which applies the common tube and the outer jacket. Each of the units must be made separately on another line and inventoried until a plurality of them can be associated together in the common tube. Because the core is generally stranded with a predetermined lay, its manufacture and the assembly of the tubes into the core involves the use of relatively heavy rotating apparatus which is undesirable from a manufacturing standpoint.
Clearly, what has been needed is a cable for optical fiber transmission which departs from those used in the past. That cable should be one which can be made inexpensively relative to present costs and which is relatively compact. Also, the cable structure should be one which inhibits the introduction of undue stresses which would lead to microbending losses in the optical fibers.
A cable which satisfies these needs is disclosed in Appl. Ser. No. 721,533 which was filed on Apr. 10, 1985, in the names of C. H. Gartside III, A. J. Panuska, and P. D. Patel. That cable includes a plurality of optical fibers which are assembled together without intended stranding to form units which extend in a direction along a longitudinal axis of the cable. A length of tubing which is made of a plastic material encloses the plurality of units and is parallel to the longitudinal axis of the cable. The ratio of the cross-sectional area of the plurality of optical fibers to the cross-sectional area within the tube does not exceed a predetermined value. A sheath system includes strength members and a jacket which is made of a plastic material and which encloses the length of tubing.
Although the above-described cable meets the aforementioned needs, efforts have continued to find alternatives which may be even less costly. This is particularly true for cable needs in buildings, both riser and plenum applications, and in loop distribution cables. Of course, if the sought-after cable is to be used as a riser cable or a plenum cable, it must have suitable smoke and flame retardant properties.
Interest has been shown in fluted core optical cables. In those, a fluted core which is made of a plastic material includes a plurality of ribs projecting radially from a center portion and a plurality of grooves with each groove being disposed between two adjacent grooves. One or more optical fibers is positioned in each of the grooves.
Prior art fluted core design cables have used cores which may be made of plastic materials such as polyvinylidene fluoride (PVDF), or polyvinyl chloride (PVC) for example. Because of the relatively low tensile strength of those kinds of fluted croes, it was found necessary to include a centrally disposed strength member within the fluted core or to wrap strength members such as ones made of KEVLAR.RTM. yarn about the exterior of the fluted core. Both remedies for the tensile strength problem add to the cost of the cable by requiring additional steps in its manufacture.
Further, what is desired is a totally dielectric cable. Such a cable which could be run from building closets to service distribution points would obviate the need for grounding connections at splice points which add to the cost of the cable installation.
Seemingly, the prior art does not include an optical fiber cable which is adaptable to a variety of environments and which includes a simplified strength member arrangement. The sought after cable should be one that may be manufactured inexpensively and that may accommodate a plurality of optical fibers in a totally dielectric structure.